Method of making a composite of paper and plastic film

ABSTRACT

Composites of paper and bioriented plastic film are made by passing a web of paperboard and a web of plastic film, with a layer of extruded molten polymer impregnating and bonding agent between the webs, through a nip. The process involves controlling one or more of the speed of the webs, the temperature of the molten polymer, the pressure or spacing of the rolls at the nip, and the rate of extrusion, relative to the porosity and surface characteristics of the paper web such that a portion of the molten polymer impregnates partially into and becomes part of the paper web and a substantial portion lies outwardly of the surface of the paper web and solidifies to form a new surface to which the film is bonded and which it is supported clear of the paper surface.

This is a continuation of application Ser. No. 08/252,665 filed Jun. 1,1994, abandoned, which is a continuation of application Ser. No.07/919,042, filed Jun. 17, 1991, abandoned, which is a continuation ofapplication Ser. No. 07/281,055 filed Dec. 9, 1988, abandoned.

FIELD OF THE INVENTION

This invention relates to composites of paper and plastic film, tocorrugated paperboard incorporating such a composite as a liner adheredto the corrugated medium, and to methods of making the composites andthe corrugated paperboard. The invention is particularly useful in thepackaging field in the form of containers, but it finds advantageousutility in the fabrication of other products, such as point-of-purchasedisplays, wall panels, posters, and the like, where a combination of theunique structural features of the invention and high quality graphics isdesired. The invention is particularly felt to satisfy a long-felt needin the area of folding cartons for packaging, whether as primarypackaging or secondary packaging, in composite form or as a liner forcorrugated paperboard. Although the composite of paper and plastic filmis highly useful and advantageous in itself, it is a particularlyimportant characteristic of the composite that it can withstand theconditions in the "double backer" part of a conventional corrugatingmachine such that it can be combined with a single faced corrugatedboard to form a double faced corrugated board without materialdeterioration of the composite, which, to my knowledge, has previouslybeen considered impossible as a practical matter. Importantcharacteristics of the composite alone or as incorporated in corrugatedpaperboard include scuff and scratch resistance, strength, stretchresistance, tear resistance, separation resistance, resistance to"checking" (cracking in a score line) in the forming of blanks forfolding cartons, surface smoothness and gloss, graphics quality anddurability, fragrance barrier quality, and moisture barrier quality.

BACKGROUND

It is known in the packaging and other arts to form a packaging materialin the form of a laminate of paper and plastic film secured together byan adhesive. For instance, particularly in the field of flexiblepackaging, various films have been laminated to paper with variousadhesives, of which polyethylene is one example. For instance, I amaware that a flexible packaging material has been made by laminating"Mylar" polyester film (a product of E. I. DuPont DeNemours & Co. Inc.,Wilmington, Del., U. S. A.) to thin high-quality paper using apolyethylene adhesive. I am also aware of Peer U. S. Pat. No. 4,254,173issued Mar. 3, 1981, which proposes a secondary container packagingmaterial (e.g., a six-pack can or bottle wrap) comprising a papermaterial laminated to a plastic film. The patent discloses variousfilms, including polyester, various papers such as kraft paper ofvarious weights, and various adhesives, including polyethylene, to bondthe film to the paper. It is disclosed in this patent and generallyknown otherwise that the film can be reverse printed with graphics priorto lamination. To my knowledge, all prior proposals have used theadhering agent only as an adhesive and, thus, have used only that amountnecessary to achieve adhesion of the two laminated components. I do notfind in the prior art any recognition or appreciation of the possibilityof using the adhering agent not just as an adhesive but rather as animpregnant and surface enhancer so as to achieve the characteristics andadvantages of the present invention.

It is a basic purpose of the present invention to provide an improvedcomposite of paper or paperboard and plastic film in which the adheringagent is used not merely as an adhesive, but rather as an impregnatingand bonding agent and as an enhancer of the paper surface by essentiallyforming a new surface. By operating under conditions which ensuresubstantial impregnation of the adhering agent into the paper, but witha substantial part overlying the surface of the paper and firmlyadhering to the plastic film, I obtain a product which is highlyresistant to separation, which enhances and protects the graphicsquality of reverse printed film or a printed paper surface, which willwithstand the rigors of the double-backer portion of a conventionalcorrugating machine, which minimizes the normal adverse effects ofscoring, cutting,.folding, etc., in the formation of a carton, and whichgenerally is a new and improved product capable of many uses.

FEATURES AND ASPECTS OF THE INVENTION

In making a composite of paper and plastic film in accordance with myinvention, a web of paper and a web of plastic film are passed into andthrough the nip of a pair of nip rolls, and a layer of molten polymerimpregnating and bonding agent is extruded into the nip between the webson the entry side of the nip. One or more of the speed of the webs, thetemperature of the molten polymer, the pressure or spacing of the rollsat the nip, and the rate of extrusion is or are controlled relative tothe porosity and surface characteristics of the paper web such that asubstantial portion of the molten polymer impregnates partially into andbecomes part of the paper web, and a substantial portion lies outwardlyof the surface of the paper web and solidifies to form an enhancedsurface, which is essentially a new surface, to which the film is bondedand on which it is supported clear of the paper surface. There iscollected from the nip rolls a composite comprising paper partiallyimpregnated with solidified polymer, a contiguous layer of solidifiedpolymer having the new surface outwardly of the paper surface, and theplastic film bonded to the new surface. In one preferred practice of theinvention, the inner surface of the film is reverse printed with desiredgraphics by any of several printing processes. However, the inventionalso provides improvements where the printing is done on the surface ofthe paper web itself, since the printed paper surface is protectedagainst scuffing of the print, and hence less ink can be used.Preferably the molten polymer comprises polyethylene, the film isbioriented, and the paper is kraft paper. A particularly preferred filmis bioriented polyester. Adherence of the components in the resultingcomposite is such that peeling essentially is not possible, in that thefibers of the paper will separate before the solidified polymer or thefilm will peel from the paper. As mentioned above, one of the mostimportant and, indeed, surprising features of the composite is that itcan be passed through the double-backer portion of a conventionalcorrugating machine to form the outside liner of a double-facedcorrugated paperboard, which typically involves movement of the exposedface of the film along a hot plate section maintained at temperatures upto about 350° F. Equally as important and surprising is the ability ofeither the composite or the corrugated paperboard of which it forms theouter liner surface to withstand the rigors of formation into foldingcarton blanks and folded cartons with little or no separation,"checking", etc. The graphics quality which can be obtained andmaintained in the end product is believed to be decidedly superior.

While the foregoing sets forth some of the basic features and aspects ofthe invention, subsidiary features and aspects of varying degrees ofimportance will be brought out in or apparent from the ensuingdescription and illustration of preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of apparatus and steps involved inpracticing preferred embodiments of the invention, including thefabrication of the composite and optional incorporation of the compositein a doublefaced corrugated paperboard;

FIG. 2 is a schematic section (not to scale) of the preferred embodimentof the composite; and

FIG. 3 is a schematic section of a double-faced corrugated paperboard inwhich the composite is incorporated as the double face liner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2 of the drawings, a composite of the invention isshown at 1. The composite includes a paperboard substrate 4, a reverseprinted plastic film 2, and an impregnating and bonding polymer 3. Asshown, the polymer has been caused to partially impregnate the paper 4so as to form a substantial thickness of impregnated paper 6. At thesame time, a substantial portion of the polymer 3 lies outwardly of thepaper surface and essentially forms a new and enhanced surface relativeto that of the paper, the reverse printed film 2 being supported on andfirmly adhered to the new enhanced surface provided by the polymer 3.The impregnation depth of the polymer into the paper is indicated at 5.The composite 1 differs markedly from the prior art of which I am awarebecause of the degree of impregnation of the polymer into the paper andthe extent to which the new enhanced surface provided by the polymerlies outwardly of the paper surface so as to prevent or at leastminimize surface characteristics of the paper affecting the reverseprinted film 2. In the conventional prior art laminates of which I amaware, the conventional adhesive does not impregnate the paper at leastto any substantial extent, and the fibrous character of the papersurface adversely affects the appearance of the film. This lattercharacteristic of the prior art can be and probably has been alleviatedto some extent by using a high quality clay-coated paper as thesubstrate. However, clay-coated paper is expensive and still would notprovide the enhanced surface provided by the overlying polymer inaccordance with the present invention. Also, clay-coated paper isrelatively less porous than conventional kraft paper such that it can bemore difficult to obtain the desired impregnation of the polymer.Without substantial impregnation, together with the overlying polymer,the product simply would not withstand the rigors of folding cartonformation while providing a high quality, commercially viable endproduct. Nor would it withstand the conditions in the double-backerportion of a conventional corrugated board making machine.

FIG. 2 illustrates a double-faced corrugated paper-board structure whichis conventional apart from the incorporation of the composite of theinvention as the double face liner, as shown at 10. The otherwiseconventional corrugated paperboard structure 7 comprises a corrugatedmedium 9 and a single face liner 8. While FIG. 3.illustrates only onecorrugated medium 9 and one single face liner 8, the combination ofwhich is typically known as single faced corrugated board, it will beunderstood that there are known in the art corrugated paperboardstructures consisting of two, three or more single faced boards glued toeach other, the single face liner of one single faced board being gluedto the corrugated medium of another single face liner so as to form abuilt-up structure. For instance, such a combination of three singlefaced boards is typically referred to as triple wall board. It is to beunderstood, therefore, that the present invention encompasses also suchbuilt-up structures in which the composite 10 forms the outer liner,such a built-up structure being exemplified by picturing FIG. 3 asincluding one, two or more additional single faced boards above andadhered to single face liner 8 or to the corresponding single face linerof a further single faced board.

Referring now to FIG. 1, there is illustrated in the upper part of thefigure a diagrammatic representation of apparatus for and the process ofmaking the composite of paper and plastic film. Collector rolls areshown at 20 and 22 to reflect the fact that various parts of the processmay be formed at different times and in different locations, althoughthe process could be continuous. From the point represented by thesecond collector roll 22, the completed composite can be passed toapparatus, schematically indicated at 23, for formation into a packageor blank or any other form, typically involving cutting, scoring,slitting, etc. Alternatively, the completed composite 1 can be passed toa corrugated board making machine for incorporation as the outside linerof a double faced corrugated board. Thus, the lower part of the figure,connected by the broken line, illustrates a typical "double backer" partof a conventional corrugated board making machine as shown in, forexample Griffith et al U.S. Pat. 3,434,901 dated Mar. 25, 1969. Fromthis "double backer" part of the machine, the double faced corrugatedboard is passed to a conventional cutting or blank forming station shownat 38 where it may be cut into sheets, formed into container blanks,etc.

As shown in the upper part of the figure, paper material, typicallyconventional kraft paper or paper-board, is led from a roll 11 through aflame treatment station 12 where the inner surface (the upper surface asviewed in FIG. 1) is flamed by a gas burner or burners to burn off loosefibers and reduce the water content. This has two effects. First, itprovides a better paper surface by burning off loose fibers, dust, etc.Second, by reducing the moisture content, it aids in the laterimpregnation of the molten polymer into the paper since the moltenpolymer seeks to replace the driven-off moisture. The flame treatment iscontrolled so as not to drive off all the moisture, or too muchmoisture, particularly from the opposite face of the paperboard, sinceotherwise the opposite face would be so dry as to reattract moisturelater and cause unwanted curling. Basically, the important point is toslightly lower and control the moisture content on the inner face so asto facilitate the desired later impregnation of the molten polymer.

From the flame treatment station 12 the paper is passed to a primerapplication station 12a where the upper surface of the paper is primered(e.g., by a roller applicator) with a primer that facilitates andenhances penetration of the molten polymer into the paper when themolten polymer is extruded into the nip of the nip rolls at thecombining station. Thus, the primer acts in the nature of a flux for themolten polymer. Such primers are known in the extrusion coating art, anda typical primer usable in the preferred process of the presentinvention is marketed in the United States by Morton Chemical Companyunder the brand name or trademark "Adcote." From the primer applicationstation 12a, the paper 4 passes into the nip between a pair of nip rolls13 and 14 where it is combined with the plastic film and molten polymer3 which is extruded into the nip from extruder 15 at a high temperature.As shown, nip roll 13 is preferably a vacuum roll, which is simply aroll having its surface covered with very small holes, the interior ofthe roll being connected to vacuum or suction such that the paper 4 onthe surface of the roll 13 can be subjected to a controllable suction toassist or control penetration of the molten polymer into the porouspaper. The use of a vacuum roll is not critical to the invention, but itcan provide an added measure of control or enhancement of impregnation.Correspondingly, nip roll 14 is illustrated as a chill roll to quicklyset or harden the molten polymer 3 to protect the film, but not soquickly as to adversely affect the desired substantial impregnation ofthe extremely hot polymer into the paper.

The plastic film entering the nip between the nip rollers starts from aroll 16 of plastic film material. The film is passed through a treatmentstation 17 where it is subjected to corona discharge treatment orchemical etching of its inner surface (the upper surface as viewed inFIG. 1). In general, the corona treatment might be described asbombardment with electrons to create minute surface crevices and cracks.The chemical etching with a solvent or acid can be considered as givinga somewhat scuffed surface. Either surface treatment has a number ofdesirable effects. Thus, there is a greater bonding area by virtue ofthe surface "roughening." It is believed that there is less meltingresistance at the peaks of the minute crevices or cracks, such that thelater heat of the molten polymer tends to melt these peaks and providebetter fusion between the film-and the polymer. The reduced meltingresistance at the peaks tends to slow dissipation of heat. Furthermore,it is believed that the film treatment has what might be termed a"rip-stop" effect, in that it tends to make the point of leastresistance against separation of the composite in the paper itself.Still further, the film treatment is believed to increase the heatresistance of the film, among other things, and hence permit morelatitude on the extrusion temperature of the molten polymer. As isalready known, the plastic film can be purchased with one or bothsurfaces already pretreated, and either treated or untreated film can beused in the present invention. Preferably even factory pretreated filmis treated again, either before or after printing, or both, sincetreating after printing does not affect the print quality, and suchpost-printing treatment is believed to enhance the ultimate bond.

From the corona or chemical etch station 17, the film passes to a primerstation 18 where any conventional primer, typical a urethane primer, isapplied to the inner surface of the film. Various such primers are knownin the art for improving the bond between a plastic film and ink. Aurethane primer is believed to avoid or minimize problems which mightotherwise occur when the molten polymer contacts the printed film.

From primer station 18, the film passes to a printing operation 19,which can be any of various printing techniques known in the art, suchas flexography, offset and gravure, using solvent-based or water-basedinks.

Although not illustrated in FIG. 1, the printed film optionally can beprimered again after printing, which, as stated previously, caneliminate or minimize the possibility of problems when the moltenpolymer contacts the ink. Thus far, this additional premiering step isnot considered critical in preferred embodiments of the invention and,therefore, can be considered as optional, although probably advantageousunder particular conditions.

As previously stated, the reverse printed film can then be passed to acollector roll for storage, transport, etc. Alternatively, it could bepassed directly from the printing station 19, as well as from thecollector roll 20, to a further corona discharge treatment or chemicaletch operation indicated at 21. This further treatment does not affectthe print quality and is believed to facilitate the bond between themolten polymer and the printed film.

The film then passes into the nip between nip rolls 13 and 14 to becombined with the molten polymer 3 from extruder 15 and the paper 4,from which nip rolls the composite passes through a further coronadischarge treatment or chemical etch station 21a for treatment of theouter film surface of the composite, and onto a collector roll 22. Themolten polymer from the extruder 15 is preferably polyethylene of lowdensity and a melt index between 12 and 15. In general, the lowestdensity polyethylene that gives a satisfactory result is used. However,where the composite is intended for use as the outer face of a doublefaced corrugated board, there should be used the lowest densitypolyethylene that will withstand the temperature and other conditions inthe "double backer" part of the corrugated board making machine.

Conditions are controlled and varied at the combining station to givethe desired product. Thus, impregnation of the molten polymer,preferably polyethylene, into the paper is controlled by controlling thetemperature of the molten polymer and the machine speed, the machinespeed being controlled by controlling the speed of the take-up orcollector roll 22. The thickness of the molten polymer passing into andthrough the nip is controlled by controlling the speed of the take-up orcollector roll, and hence the rate of travel of the paper and filmthrough the nip. Additionally, the spacing of the nip rolls can bevaried. In a preferred arrangement, one of the nip rolls is springloaded or pneumatically loaded, and the spring pressure or pneumaticpressure can be controlled. In general, although practically any of theconditions at the combining station could be controlled and varied asneeded, including the pressure or spacing of the nip rolls, thethickness, temperature and viscosity of the molten polymer, the speed ofthe webs, and the rate of extrusion, generally it suffices tocontrollably vary only three conditions, these being the take-up rollspeed, the temperature of the molten polymer, and the spring orpneumatic pressure on the adjustable nip roll. Theoretically it would bepossible to vary the speed by controllably varying the speed of the niprolls, but this would be an unnecessary complication relative to varyingthe speed of the take-up roll.

The schematically illustrated apparatus for making the composite shouldbe considered as merely typical, although presently preferred. Ingeneral, the process of making the composite can be carried out onproperly controlled conventional machinery normally used for merelylaminating paper, or machinery normally used for extrusion coating paperif modified to additionally handle the film. In the typical practice ofthe process, the treated and reverse printed film is taken up oncollector roll 20, which is thereafter transported to and mounted on thecombining machinery, appropriately modified as required. As previouslymentioned, this could be conventional machinery for extrusion coatingpaper modified to handle and treat the plastic film.

The primary purpose of the corona discharge treatment or chemical etchtreatment indicated at 21a is to facilitate gluing of the outer surfaceof the film to itself or some other surface in the fabrication of boxes,containers, etc. Thus, it will be understood that the treatment atstation 21a is of the outer surface of the film part of the composite.

The package/blank forming operation indicated at 23 may be anyconventional operation for forming carton blanks, cartons, boxes,containers, or simply cutting the composite into individual sheets foruse as desired.

To combine the completed composite as part of a corrugated structure,the roll of composite can be mounted in a conventional corrugatingmachine as the supply roll for the outside liner of double facedcorrugated board, as diagrammatically indicated in the lower part ofFIG. 1. As seen in the lower part of FIG. 1, which shows a typical,prior art, double backer operation, single faced corrugated board 24,consisting of a web of corrugated medium 24a having a coextensive web ofliner 24b bonded to tips of the corrugations on one side thereof, istrained over a preheater drum 25. The single face 24, after having itstemperature raised to perhaps 150°-200° F. by the preheater drum 25, isthen passed over an adhesive applicator 26 and into the combiningsection generally indicated at 27. The composite 1, constituting a webof outside liner, is also trained over a preheater drum 28 and thenceinto the combining section 27 where it is pressed against the adhesivecovered flute tips of the single faced board. Of course, the paper faceof the composite is the upper face as viewed in the lower part of FIG.1, such that the corrugated medium 24a is pressed against the paper faceof the composite 1. The combining section consists, basically, of twoparts: a hot plate section 29 and a draw section 38. Heat is applied inthe hot plate section by a series of steam heated chests 31 having theirupper portions aligned to provide a heated, substantially continuoussurface. An endless belt 32, trained about a pair of driven rollers 33and having a series of smaller rollers 34 bearing on its lower reach,serves to press the single faced board 24 and composite 1 toward thesteam chest 31. A second belt 35, trained about driven rollers 36 andpressure rollers 37, cooperates with the downstream portion of the belt32 to grip the assembled single faced board and composite outside liner1 and draw them through the double backer apparatus. The completeddouble faced board is then passed to an appropriate operation stationindicated at 38 for cutting, box blank forming, or whatever operation isdesired. Apart from the incorporation of the novel composite as theoutside liner, the lower portion of FIG. 1 may be regarded asconventional, and, indeed, this is one of the great advantages of theinvention in that this is the first instance of which I am aware inwhich a double faced corrugated board can be made on a conventionaldouble backer corrugating machine while providing an outside linerhaving the features and characteristics of the composite of the presentinvention. Thus, the invention provides not only a new and advantageouscomposite of paper and plastic film useful in itself, but also providesa composite which can successfully withstand the conditions involved inpassing through the double backer part of a conventional double facedboard corrugating machine as the outside liner.

In the finished composite, adherence of the original components is suchthat they are essentially inseparable, that is, peeling is practicallyimpossible. The weakest point against separation is in the unimpregnatedpart of the paper, and the fibers of the paper will separate before themolten polymer, preferably polyethylene, will peel from the paper, andtypically before the plastic film will peel from the polyethyleneimpregnant and bonding agent. It is indeed surprising that the compositecan go through a double backer corrugator with no or little damage tothe film or the polyethylene impregnant and bonding agent, consideringthat the heat plates of the corrugator typically involve temperatures ofup to 350°F., a temperature at which regular adhesives will not stand upand, indeed, a temperature at which it is believed that the polyethyleneof conventional "poly mounted" laminates will not stand up. Furthermore,polyethylene being moisture resistant, the new composite withimpregnated polyethylene is highly resistant to separation even whenwet.

Successful runs of the composite have been made in a Langston 87" XBcorrugator, 1965 model, the heat table in the double backer being at atemperature of approximately 340°. The composite should run well insimilar conventional machines under normal commercial operatingconditions or, perhaps in some instances, with minimial deviations fromnormal commercial operating conditions. It is difficult to attribute theability of the composite to successfully pass through a double backercorrugator to any particular feature or features. However, a tentativetheory is that the substantial amount of polyethylene impregnating andbonding agent present in the composite softens slightly in passing overthe hot plate, so as to act as a cushion and reduce or eliminatescuffing of the film as it drags over the hot plate. Also, it may bethat the paper and the substantial amount of polyethylene impregnatedtherein act as a heat sink to prevent excessive softening or bubbling ofthe polyethylene overlying the paper and forming the new or enhancedsurface to which the film is bonded.

In the making of the composite, conditions should be controlled suchthat in the completed composite there generally is at least about 0.5mil of polyethylene between the reverse printed film and the surface ofthe paper. This generally ensures that paper fibers, with or without thepreferred flame treatment operation, do not contact the inner surface ofthe film. In general, in the extracted composite the polyethylene shouldbe generally uniform over the area of the composite in amount of atleast about five pounds per thousand square feet of composite, withpreferably at least about three pounds of polyethylene per thousandsquare feet being generally uniformly present between the printed filmand the surface of the paper to form the new and enhanced surface. Wherethe paper is of the order of forty-two pound (forty-two pounds perthousand square feet) liner board, the preferred minimum totalpolyethylene is at least about six pounds per thousand square feet ofcomposite, with a preferred range being about nine pounds to fourteenpounds per thousand square feet of composite. Preferably the extrudedmolten polymer, preferably polyethylene, passes into the nip at a ratecorresponding to a continuous thickness of at least about one milrelative to the speed of the webs, and advantageously this continuousthickness should be at least about 1.5 mils. As previously mentioned,impregnation of the polyethylene or other polymer into the paper may becontrolled by controlling the temperature of the molten polyethylene andthe machine speed. In general, decreasing the temperature of thepolyethylene decreases impregnation. Therefore, the polyethylene shouldbe kept at a high temperature, typically about 600° F. or higher. Ifclay coated paperboard is used in the process, it is generally necessaryto use a very high temperature for the polyethylene so as to ensuremigration through the clay coating and impregnation into the paperboard.The clay coated paperboard, even though it has an improved surfacerelative to uncoated kraft paper, generally should be flame treatedabout the same as uncoated kraft paper, but possibly slightly less. Ifusing clay coated paperboard, it is important not to drive off too muchmoisture, partcularly from the opposite face of the paperboard, sinceotherwise the opposite face will reattract moisture and cause curling,possibly more so than for uncoated kraft. While usable in the-invention,clay coated paperboard is not preferred because of its expense, onefeature of the invention being the provision of a high quality productfrom a low grade or relatively inexpensive paper or paper-board. It isalso to be noted that in general machine finished paper does not work aswell as plain kraft paper or paperboard. This is believed to be becausemachine finished paper has a polished surface, and is not sufficientlyporous for the most advantageous practice of the invention. In general,natural kraft paper, unbleached and uncoated, works well in the practiceof the invention.

In general, the printing of the film will involve continuous coverprinting where high quality graphics are desired, although this is notcritical to the invention for some uses where high quality graphics arenot needed or where only the structural features are desired. However,as a variant of the invention as previously described, high qualitygraphics could be obtained by printing the film without continuous printcover, that is, with some unprinted areas. The film so printed thencould be extrusion mounted on metallized film, which then would becombined with paperboard to form a composite as previously described.The metallized surface of the metallized film should be toward thepaperboard.

Apart from the packaging field, a principal use of the composite is inthe manufacture of wall paneling by laminating the composite towallboard such as particle board or plywood, the paper surface beinglaminated to the wallboard. The composite for such use is made in thesame general manner previously described, except that a lighter weightof paper can be used in the composite, such as ten pound or twelve poundkraft paper. This results in a scuff resistant panel of high graphicsquality. Where high gloss is undesired, delustered film such asdelustered "Mylar" film may be used. Alternatively, polypropylene filmcould be used. The film need not be, but preferably is, bioriented sincethis produces a better quality product, and avoids fabrication problemsthat might arise because of the relatively less stability of unorientedfilm.

An outstanding characteristic of the comnposite of the inventionrelative to conventional laminates is its resistance to separation,"checking," cracking, etc. when being scored to form box blanks, forinstance, or when being folded into completed boxes or other containers.Scoring involves substantial compression in a small area, and in manylaminates can result in separation or delamination, which does not occurwith composites in accordance with the present invention. The reason forthis is difficult to pen down to a particular feature, but it isbelieved to result from a combination of the substantial impregnation ofthe polyethylene into the paper and the overall cushioning effectprovided by the impregnated and unimpregnated polyethylene. No doubt thepreferred bioriented polyester film contributes to this also. Ingeneral, containers of which the composite of the invention forms theouter surface have a smooth, high-gloss surface that will not separateor "check" in the corners after scoring.

Cartons or boxes made from the composite per se or in its corrugatedform not only provide high strength and resistance to crushing, but alsoare moisture proof or resistant and retain fragrance, characteristicswhich are extremely important in primary container packaging forproducts such as laundry detergents, soap, etc.

The paper or paperboard used in the composite of the present inventiongenerally may be any paper suitable for folding cartons or corrugatedboard or as a substrate for laminating to a backing such as wallboard.The preferred paper is kraft paper of a weight known as liner board orpaperboard. As is well-known in the art, kraft paper is paper producedby a chemical cooking process using sodium hydroxide and sodium sulfide,and there are many different types of kraft paper manufactured withvarious additives and treatments for various applications. Natural kraftpaper generally refers to kraft paper which has not been bleached ordyed. Of course, paper itself refers to a web of cellulosic fibers insheet form. The invention can also make good use of reprocessed paper,that is, not virgin kraft paper. In general, the heaviest paperpresently contemplated as useful in the invention is twentytwo pointkraft liner board having a weight of about ninety pounds per thousandsquare feet. The term "point," as generally used in the industry andherein, means a thickness of one-thousandth of an inch for each point.For composites to be incorporated in a corrugated structure, a preferredpaper is natural kraft paper (unbleached and uncoated), twenty-sixpounds per thousand square feet or heavier, and commonly about ninepoint. Other useful paperboard for corrugated structure incorporation istwenty-three pound paper, with thirty-three pound to forty-two poundpaper being also typically useful. For some uses of the composite itselfas secondary packaging, fifty pound, nine point kraft would be a typicalexample. In general, one should use the least expensive and lowest gradepaper that results in a composite having the necessary characteristicsfor the particular project.

The much preferred film for use in the composite of the invention isbioriented polyester such as DuPont's "Mylar" film, which is a strong,tough, clear plastic film made principally from polyethyleneterephthalate and used widely in packaging, particularly flexiblepackaging. Polypropylene might be used where less scuff resistance isneeded, but the polypropylene and polyethylene impregnant and bondingagent-preferably should be cross-linked by radiation after formation ofthe composite. Radiation cross-linking itself is well-known in the art.The presently preferred "Mylar" film is one-half mil (fortyeight-gauge).Where the composite is to be used in a corrugated structure, the filmshould be a bioriented film of high heat resistance sufficient tosuccessfully go through the double-backer part of a conventionalcorrugator having a heat plate or table operating at about between 250°and 350°.

The preferred impregnating and bonding agent is polyethylene, typicallya low density polyethylene having a melt index between twelve andfifteen. For use in a composite to be incorporated in a corrugatedstructure, one should generally use the lowest density polyethylene thatwill successfully go through the double-backer part of a conventionalcorrugated board making machine. Examples of suitable polyethyleneresins are Eastman 1390, of 0.915 density and melt index of fifteen,manufactured by a subsidiary of Eastman Kodak Company. Another exampleis DuPont's "Alathon" 1570, a low density polyethylene resin known foruse in flexible packaging.

As previously stated, the molten polyethylene preferably is extruded at600° F. or higher, but this should be varied as needed to achieve thedesired substantial impregnation into the paper. In general, theextrusion temperatures used are substantially higher than would benecessary merely to adhere two components together in a conventionallaminating process, the higher temperatures facilitating theimpregnation. As an example, if the molten polyethylene is of 1.5 milscontinuous thickness out of the extruder, conditions should bemaintained such that at least about one-half mil will penetrate andimpregnate into the liner board.

As a typical example of making a composite for use as the outside linerfor a double-faced corrugated board, one could use one-half milbioriented polyester film (typically "Mylar"), twenty-three pound paper,and at least one mil thickness of polyethylene out of the extruder andthrough the nip. As another typical example, the composite would involveforty-eight gauge (one-half mil) bioriented polyester film, a minimum of1.5 mils thickness polyethylene out of the extruder and through the nip(of which at least one-half mil should penetrate and impregnate theliner board), the polyethylene being extruded between about 600° and640° F. and being of low to medium density, and twenty-six pound lowdensity kraft paper of nine point thickness. An alternative paper couldbe thirty-three pound to forty-two pound kraft paper. As an example of acomposite for use in secondary packaging, such as a "six-pack" or"twelve-pack" for cans or bottles, there could be used sixteen point,fifty pound kraft paper, forty-eight gauge bioriented polyester film,and two mils continuous thickness of polyethylene out of the extruderand through the nip.

Although the illustrated embodiment involves reverse printing of thefilm, and this technique generally will be used where the highestquality graphics are desired, it is to be understood that the inventionis advantageous also where the printing is done on the paper surface,that is, printing the paperboard or liner board instead of the film. Aprincipal advantage of the invention in such embodiments is that theprinting on the paper is well protected against scuffing, and hence lessink can be used. In typical paperboard printing, it is necessary to laydown a fairly heavy coat of ink to allow for scuffing and the like. Withthe printed paperboard well protected by the polyethylene and the filmin the present invention, this is not a problem. Thus, one can use lessink while achieving better quality printing, using, for example, a 120line screen. In general, where the paperboard is to be printed, it ispreferred to use clay coated kraft paper or solid bleached sulfate kraftpaper or paperboard or linerboard, typically known in the art as SBSkraft liner-board. Both products have an improved surface relative tountreated kraft paper. In such embodiments of the invention, not only isthere an advantage in the initial printing of the paperboard and in theprotection of this printing, but there. is also an advantage inappearance arising from the smoothness of the film supported on the newor enhanced polyethylene surface, such that the characteristics of thepaper do not carry through to the film, and hence there is improved filmgloss where desired.

As mentioned previously, it is known in the art to laminate paper andpolyester film, including bioriented polyester film, using apolyethylene adhesive, in, for instance, flexible packaging. It is alsoknown in the art to produce a laminate of polyester and paperboard bycoextrudina polyester and polyethylene adhesive onto paperboard whichhas been pretreated by flame priming, the polyethylene adhesive layerbeing thinner than the onehalf mil polyester outer layer, as describedin Thompson U.S. Pat. No. 4,455,184, issued Jun. 19, 1984. KozlowskiU.S. Pat. No. 4,064,302 relates to flexible, semi-rigid fabricatingmaterial, and mentions "Mylar," among others, bonded to paper orpaperboard by an adhesive coating of thermoplastic polyethylene. Thepreviously mentioned Peer U.S. Pat. No. 4,254,173 discloses a secondarycontainer packaging material for use in six-pack can wraps, etc.comprising a laminate of a paper material laminated to a plastic film,which paper may be natural kraft paper among others, which film may bepolyethylene terephthalate, among others, the film being reverseprinted, a mentioned adhesive being a polyethylene resin, among others.However, these known or proposed products do not respond to thestructural and appearance features of the present invention and are allbelieved to be distinctly different from the present invention.

In the foregoing, I have described and illustrated the inventiveconcepts with reference to illustrative and presently preferredembodiments of my invention. However, the scope and substance of myinvention are as set forth in the ensuing claims as interpreted in thelight of the foregoing description and illustrations, and it is intendedthat the claims be construed as including alternative embodiments,except insofar as limited by the prior art.

I claim:
 1. A method of making a composite of paper and plastic film,comprising passing into and through the nip of a pair of nip rolls a webof kraft paper and a web of plastic film of bioriented polyester,extruding a molten plastic impregnating and bonding agent ofpolyethylene at a temperature of at least about 600° F. into the nipbetween said webs of a thickness such that the combined thicknesses ofthe molten plastic, the paper web and the plastic film web passing intothe nip are greater than the least dimension of said nip; the porosityof the paper, the moisture content near an inner surface of the paper,the pressure of the nip rolls, and the thickness, temperature andviscosity of said molten plastic agent being such that part of saidmolten plastic agent impregnates partially into and becomes part of saidpaper web and a substantial part of the plastic agent extends outwardlyof the inner surface of the paper and forms a new solidified surface onwhich the plastic film is supported and to which it is firmly bonded,wherein about three pounds of polyethylene per thousand square feet isgenerally uniformly present between said film and the inner surface ofthe paper, and extracting from the nip rolls said composite consistingof paper at least partially impregnated with the plastic impregnatingand bonding agent so as to be inseparable therefrom without destructionof the paper, the new surface of the plastic agent clear of the innersurface of the paper, and the plastic film web supported on and bondedto the new surface of plastic agent clear of the inner surface of thepaper, the thickness of the composite being less than the combinedthicknesses of the paper web, the extruded layer of molten plastic, andthe plastic film web, passing into the nip, and the polyethylene agentbeing extruded generally uniformly over the area of the composite in anamount of at least about six pounds per thousand square feet.
 2. Amethod as claimed in claim 1 wherein the inner surface of said plasticfilm is provided with a decorative surface coat before passing into saidnip.
 3. A method as claimed in claim 2 wherein the inner surface of saidplastic film is printed before passing into said nip.
 4. A method asclaimed in claim 1 wherein said molten plastic impregnating and bondingagent is polyethylene, said plastic film is bioriented polyester film,and said paper is uncoated kraft paper.
 5. A method as claimed in claim1 wherein the extruded layer of molten polyethylene passing into the nipis of substantially greater thickness than said bioriented polyesterfilm and substantially less thickness than said paper web.
 6. A methodas claimed in claim 1, further comprising providing a single-facedcorrugated paperboard having a corrugate medium with flute tips on oneside of said medium and, adhering a paper surface of said composite tothe flute tips on one side of the corrugated medium so as to form adouble-faced corrugated paperboard.
 7. A method as claimed in claim 6wherein said composite is adhered to the flute tips of said corrugatedmedium by adhesively applying the paper surface of the composite to theflute tips of the single-faced corrugated paperboard in a double-facercombining section of a corrugating machine having a hot plate sectionsuch that the composite forms the outside liner of a double-facedcorrugated paperboard and the exposed outer face of said film movesalong the hot plate section in passing through the machine.
 8. A methodas claimed in claim 7 wherein said double-faced corrugated paperboard issubsequently subjected to one or more of cutting, scoring, slotting andslitting.
 9. A method as claimed in claim 1 in which the moisturecontent of the inner surface of the paper is controlled by flametreating the inner surface of said paper before passing into said nip soas to remove some of the loose or protruding fibers and lower the watercontent so as to provide a smoother paper surface and minimize papersurface fibers extending to the new surface of the plastic agent.
 10. Amethod as claimed in claim 1 wherein said extruded molten polyethyleneis extruded into said nip at a temperature of approximately 600° F., anda nip roll contacted by the film is a chill roll.
 11. A method asclaimed in claim 1 wherein sald polyester film is pretreated on an innersurface by a pretreatment selected from corona treatment and chemicaletching.
 12. A method as claimed in claim 1 wherein the inner surface ofthe film is treated with a urethane primer.
 13. A method as claimed inclaim 1 wherein operating conditions are such that in the compositethere is at least about 0.5 mil of polyethylene between said film andthe inner surface of the paper.
 14. A method of making a composite ofpaper and film comprising passing into and through the nip of a pair ofnip rolls a web of kraft paper and a web of plastic film of biorientedpolyester; extruding a layer of molten polymer impregnating and bondingagent into the nip between the webs on the entry side of the nip;controlling one or more of the speed of the webs, the temperature of themolten polymer, the pressure or spacing of the rolls at the nip, and therate of extrusion, relative to the porosity and surface characteristicsof the paper web, and controlling the moisture content near an innersurface of said paper web, such that a substantial portion of the moltenpolymer impregnates partially into and becomes part of the paper web anda substantial portion lies outwardly of the surface of the paper web andsolidifies to form a new surface to which the film is bonded and onwhich the film is supported clear of the inner paper surface; andcollecting from the nip rolls said composite comprising paper partiallyimpregnated with solidified polymer agent, a contiguous layer ofsolidified polymer agent having said new surface outwardly of the paperinner surface, and the plastic film bonded to said new surface, and thepolymer agent being generally uniform over the area of the composite inan amount of at least about nine pounds per thousand square feet.
 15. Amethod as claimed in claim 14 wherein the inner surface of said film isreverse printed.
 16. A method as claimed in claim 14 wherein saidpolymer agent comprises polyethylene.
 17. A method as claimed in claim16 wherein said polyethylene is extruded into the nip at a temperatureof at least about 600° F.
 18. A method as claimed in claim 15 whereinthe reverse printed film is pretreated on its inner surface by apretreatment selected from corona treatment and chemical etching.
 19. Amethod as claimed in claim 14 wherein said film is pretreated on aninner surface by a pretreatment selected from corona treatment andchemical etching.
 20. A method as claimed in claim 14 wherein a nip rolladjacent said paper web is a vacuum roll imposing suction on said paperweb, and a nip roll adjacent said plastic film web is a chill roll. 21.A method as claimed in claim 14 wherein the operating conditions aresuch that the extruded molten polymer passes into the nip at a ratecorresponding to a continuous thickness of at least about 1.5 mil.
 22. Amethod as claimed in claim 21 wherein the operating conditions are suchthat the solidified polymer agent lying outwardly of said inner surfaceof the paper is of at least about 0.5 mil thickness.
 23. A method asclaimed in claim 14 further comprising treating the inner surface ofsaid paper before passing into said nip so as to remove some of theloose or protruding fibers and surface dust and lower the water contentso as to provide a smoother paper surface, enhance impregnation by themolten polymer to replace the lowered water content, and minimize therisk of paper surface fibers extending to the polymer surface.